Strength recovery of dispersion hardened alloys



N. J. GRAN-r 3,139,682

y July?, 1964v s'mfamcm RECOVERY oFfnIsPEvsIon HARDENED ALLYs Filed June24. l1960 l IN1/Enron.

4009/0045 n! @e4/V2" f United States Patent O 3,139,682STRENGTH-RECOVERY F DISPERSION f y HARDENED ALLOYS Nicholas J. Grant, 10Leslie Road, Winchester, Mass. 1 Filed June 24, 1960, Ser. No."38,521

1 Claim. (Cl. 29552.2)l

This invention'relates to a method for the recovery of high`^temperatureproperties of certain lwrought metal compositions of the class based-`on the metal-metal oxide system in which a matrix metal or alloy 'hasdistributed fractory oxide phase.

In particular the invention is directed to 4the treatment of theaforementioned class of wrought metals which have been subjected to thedeleterious effects of elevated temperatures wherein the-physical andother properties of said metalshave been substantially impaired. Myinventionis applicable tothe recovery of those metals of theaforementioned class which have been impaired by heating to very hightemperaturcsup to below the melting point `of the matrix` metal whereinrecovery and softening has occurred; to those metals which have beensubjected to brazing or solid state welding in which the properties atthe braze or weld have been deleteriously affected; to those metalswhich have been heated to above the transformation temperaturevduringfabric-ation o r other treatments in which there has been a' dissipationof stored energy with a consequent loss in strength; and to othersituations in which prevailing high temperatures have had an adverseeffect upon lthe physical properties of the metal.

Mechanical working, such as cold working, is`A known to increase thehardness and strength of yconventional metals and 'alloys and it is. notuncommon to provide commercially metals having varying-degrees of strainhardening depending upon the ultimate use to which the metals are to beput.` Generally, strain hardened metals, particularly metalsincapable ofbeing heat treated to higher hardnesses, are limited to room temperatureapplications or totemperatures not too far above room temperature 'forthe reason that metals which have been -strain `hardened by cold workingare not stable, but tend to revert to the strain freestate when heatedto temperatures which produce a softening effect on metals. 't

When ametal is subjected to cold working and therea'fter heated tosuccessively higher? temperatures until-` softening setsin,rthe,temperature at which this occurs is referred to as thetemperature of reerystallization. The temperature of softening isrelated to the degree of strain hardening, that is the higher the amountof cold working, the lower is the temperature of softening orrecrystallization. `For example, a copper composition `cold worked 3%(i.e. reduction by cold rolling) may exhibit a temperature of softeningin the neighborhood of about 450` i C. while the same`compostion coldrolled to a reduction of y20% may have a lower temperature of softeningof about 400 C. `At 50% coldpwork, the softening temper.-

rature maybe about 3,5,0.1. whil e .at 95%' cold work, the

temperature may vfall belojkyl BOQCL f t Because of theforegoing-*limitations it has not been possible to` utilize effectivelymechanical working as a t A means of recovering physical or otherproperties of metals -or alloys which have been impaired throughoverheating.

However, I have found that as to `certain metal compositions, I canemploy mechanical working, i.e. strain f hardening, not only to recoverproperties lost through overheating, but in many instances to improveupon the properties then existing inthe metal. By mechanical working, Iinclude cold working as well as any working at temperatures above roomtemperature at which deformationalenergy is stored in the worked metal.

Ihave'ound, `for example, that if the metal was origtherethrougha tinedispersion of `a hard particle or re- Patented 'July 7, 1964 lostthrough aggravated heating ata very high temperature, wellabove thenormal recrystallization temperature of the4 base metal or because ofoverheating due to 5 welding or b razing, l can bring the metal back toits opti-v mum condition by strain hardening without themetal tending torevert back to the strain free state at elevated temperatures when therecrystallization temperature of the unsupported matrix is exceeded. l'nother words, the

strain hardened metal of my invention retains its high strengthproperties on heating to relatively high tempera'- ture levels withlittle dissipation of the stored energy. even above temperatures atwhich comparable metals or alloys generally recrystallize and soften.Moreover, whereas in conventional metals, e.g. cold worked copper, thetemperature of recrystallization decreases substantially with increasein cold work, the compositions with which I work do not appear to be soaffected and are relatively stable at elevated temperatures over a rangeof applied cold work. j v

It is the objectof the invention to provide a method of providingwrought metal articles in which applied cold' work is utilized torecover the strength properties of the metal without affecting thestability of the metal in resisting t recrystallization at elevatedtemperatures.

Another object is to provide a method whereby certain metal compositionswhich have been softened by brazing or welding, such as spot, seam, buttor flash welding, or other forms of overheating can be recovered bymechanical working, whereby the metal thereafter will resistrecrystallization at exceedingly high temperatures and substantiallyretain its stored energy.

These and other objects will more clearly appear from the disclosure andthe appended drawings, wherein:` FIG. l depicts graphicallyA theimproved rupture life obtained for a coldworked alloy comprising copperand silica;and 4 l `FIG. 2'is similar to FIG. -1 but exhibits theimproved rupture life obtained for a cold worked alloy comprising copperand alumina.

`I-have found thatV a metal containing av substantially uniformdispersion of inely divided substantially insoluble refractory material,e.`g. refractory oxide, and whose properties have been impaired bysubstantial overheating, can be physically processed toconfer improvedhigh temperature properties, e.g.` resistance to creep, bymechanicallyworking said metal. Unlike lthe mechanical `working ofconventional metals and alloys, I tind that the mechanical working of mymaterial does not affect 5d adversely the recrystallization temperaturebut that, on

the contrary, the material so processed resists recrystalliza-v tion atelevated temperatures well above the-normal recrystallizationtemperature of the matrix metal and appreaching the melting point in thevalloys dispersion 5'5 strengthenedtoanfoptimu'rm t.

1 ,IA meta 'regains Phenomenon has been Iz-pointed out hereine workedcondifofilnd,that when F Ni lener `is obtained in several wa'ysgforexample', either-by mechanically mixing a substantially inert oxide,eig. A1205', SiOg, T1102, ZrO, and the like, with matrix metal powderfollowed by consolidation of the mixture into a wroughtl shape,. or byinternal oxidation. e

In producing the material by internal oxidation, a refractoryoxide-forming metal, e.g.,aluminum, is alloyed asa dilute solution witha matrix metal, such as copper, and the alloy in particulate form, e.g;minus 44 microns, subjected to an oxidation treatment adapted toselectively oxdzealuminum t'o a dispersion of A1203 fol- .lowedbyconsolidation lof the' particles to a wrought temperatures shown inTable I.

Table I Wt. Voi. Percent Oxidation Oxide Iar- Alloy No. Percent OxideTemp., ticle Element fC. Radius. A.

powders involved the surface oxidation of4 the powders to form a surfacecoating rich in Cu3O (and also containing some solute oxide) by heatinga given amount of powder in arneasured amount of oxygen at about 450 C.The oxygen from the surface oxide was then diifus'ed into the sample byheating at the desired temperature, e.g. 650 C., under substantiallyinert-conditions. This method was found vadequate for obtaining up toabout 12% by volume of solute oxide within `the matrix metal.

As an--example of one method employed in effecting the internaloxidation of the alloy powder, the surface oxidized powder, for example450 grams, is sealed in a 1.5 inch diameter tube by iiattening'of theends. The tube is placed in a large `mutlie furnace held at the desiredtemperature, e.g. 650 C. or 750 C. for a time, determined by pilottests, suicient yto obtain a uniform dispersion of 'solutefmetal oxidein each of the matrix metal particles. i t l The internally oxidizedmatrix metal powder is thereafter hydrogen reduced at an elevatedtemperature,e.g.

450 C. for'one hour.' to rid the surface of each particle of` copperoxide and then packed by vibration in a copper container of about 1.4inch I.D. by 4.5 inch long to achieve a pack density of about 50%.Thecontainer is evacuated trusion.

f'course, it will be appreciated that the foregoing met od is by way ofillustration and that many variations of accomplishing the same resultmay be substituted. For example, the internal oxidation could also beachieved by treating the alloy powder at a low partial pressure ofoxygen (e.g. subatmospheric pressure) at which copper and sealed andmade I'ready for direct ex'- does not readily oxidize and the solutevmetal oxidized by diffusion `of the oxygen into'the matrix metalpowder', However, I prefer in carrying out my invention to utilizementioned` at 760 CZ- iising a speed`of about 55 inches minute tof'giv'ean extrusion ratio of about 28 to l. Rods of approximately 0.25 inchdiameter by four feet in lengthwere obtained from the alloys.

Alloy No. l'was partially recrystallized n the asextruded condition,that is 'part of the grains making up the copper matrix metal wereequiaxed, indicating that some softening had occurred during hot workingby extrusion. After subjecting thisalloy to aggravated heating at justbelow the melting point of copper, 10-hoursv at l050 C., the. matrixmetal was 100% recrystallized. The metal was thencold worked 25% andsubjected to a rupture life creep test comprising heating the alloy atvarious stresses at 450 C. until rupture occurs. The results of the testindicated that the alloy in the cold worked condition was markedlysuperior at 450 C. to

crystalliz'ed condition and maintained these properties for prolongedperiods of time at 450 C. without further recrystallization occurring.It will be noted from FIG. 2 that the material cold worked 10% exhibitedmarked improvement at 'high temperatures over the partiallyrecrystallized material. Similar trends were noted with respect to alloyNos. 2, 3 and 4. It is thus apparent that strain hardening in this classof alloys does not lower the temperature of recrystallization as in theconventional alloys but. that, onthe contrary, the stored energy isretained at elevated temperatures.

Likewise, similar results 'were obtained on dispersion hardened materialproduced by the mechanical mixing of powders. In producing alloys bymechanical mixing as disclosed in copending application Serial No.24,971, tiled April 27, 1960,matrix copper powder about 1 micron in sizeis mixed separately with speciiiedamounts ot finely divided SiO, orA1203, preferably of particle' size ranging from about 30 to- 150 timessmaller than the matrix metal powder, e.g. 0.02 micron. Dry mixing isused by utilizing i a Waring Blendor at a speed of about 15,000revolutions per minute. Thel mixing is carried out for about 15 minutesand thenfurther mixed by spatulationv on a sheet of clean paper for sfew minutes, the procedure with the Blender andthe subsequentspatulation being'repeated about four times. t

The blended powder mixture is thereafter subjected to a reducingtreatment in dry hydrogen fora minimum of tive hours at a temperature ofabout 430 C. to insure clean particle surfaces for subsequenceconsolidation of the mixture into wrought shapes. Each batch of themixed powders is introduced into a rubber tube supported withinaperforated steel canister about two inches in diameter, one end of therubber tube being rubber stoppered at the start. `After the powder isintroduced, a second rubber stopper havingtin communication therewith ahypodermic needle is inserted, avacuum connection being made through theneedle to remove the air from within powder mass. After completion ofevacuation, the needle is removed and the canister assembly subjected tohydrostatic pressure hat about 30,000 p.s .i. to yield compacts 1.4inches'lin diameter and 2.5`inches long.

rnpacts; areQthensjubiected to sintering in dry hydro for a minimum of10 hours at 830 C. After that; they are each canned by insertion in apure'copper container and welding vacuum tight followed by extrusion thefirst method described as I find that this ymethod.leiid itself tocaseof control andreadily reproduceabl iat an elevated temperature. Theextrusion ratio used is about 16 to 1,'although the ratio could haveranged from' 12|to 1 to V30 to 1.

Shapes produced by the foregoing manner, in which at least some ofthegrans of the matrix metal re equiaxed,

lplatinur'n, etc.

at elevated temperatures for prolonged periods of 'time withoutrecrystallization or without substantially losing product exhibited a'100 hour rupture strength at 1100 F. of about 11,000 p.s.i.However,.after cold working the same composition about 25 the 100 hourrupture stress at 1100" F. was raised from 11,000 p.s.i. to 14,000p.s.i., anincrease of over 27%, the alloy, as cold worked, ex-v hibitingparticularresistance to creep at elevated temperatures while resistingrecrystallization. A A It is' apparent that the invention is applicableto wide variety of dispersionstrengthened ductile matrixmetals, be theybased on alloy matrices or pure metals capable o'f being deformed into avariety of shapes. The lower melting metals Mg, Zn, Cd, Al,l In, Pb, Snand their alloys containing oxide dispersoids may be so treated. Thehigher melting metals comprising the copper group metalsV (Cu, Ag, Au)and their alloys, the platinum group metals (Pt, Pd, Ru, Rh, Ir, etc.)and their alloys, the iron group metals (Fe, Ni, Co) and their alloysmay also be treated and so zinc; 90% copper and 10% zinc; 60% copper and40% zinc; 71% copper, 28% zinc and 1% tin; 65% copper, 17% zinc and 18%nickel; 90% silver and 10% copper; up to nickel and the balance silver;70% gold and the balance palladium; 69% gold, 25% silver and 6% Examplesof'iron group alloys include: certain steels; 64% iron and 36% nickel;31% nickel, 4 to 6% cobalt,

`and the balance iron; 54% iron and 46% nickel; 99%` nickel and thebalance cobalt; 68% nickel and 32% copper, etc.,

Examples of platinum group alloys are as follows: patinum-rhodium alloyscontaining up to 50% rhodium; platinum-iridium alloys containing up to30% iridium; platinum-nickelA containing up to 6 or 10% nickel;platinum-palladium-ruthenium containing 77% to 10% platinum, 13% to 88%palladium, and 10% to 2% ruthenium; alloys of palladium-rutheniumcontaining up to8% ruthenium; 60% palladium and 40% silver, etc. t 4

Examples of refractory oxides which maybe used in producing dispersionhardened alloys in accordance with the` invention are SiOg, A1203, MgO,BeO, Zr03, T102, ThO, and oxides of the rare earth metal group,.such asoxides of cerium, lanthanum, neodymium, etc. Such refractory oxides arecharacterized as having a melting point above the melting point of thematrix metal and generally above 1500 C. In addition, these oxides arecharacterized by a negative free energy of formation at about C.

. ple, dispersion hardened iron containing about 8% by volume of A1203was produced by extruding in the austeniticV temperature range, e.g.about 1900? C. The extruded l from about 0.01 to 0.1. Where thedispersion strengthened metal is produced by powder mixing, the metalpow'- der should be larger than 'the oxide and preferably to 150 timeslarger..

The amount of working applied to dispersion hardened material willdepend upon the relative softness of the alloy and the amount ofdisperse' phase present. Generally, the amount of working will beinversely related to I the vol.,percent of disperse phasepresent, thatis, the higher the amount of disperse phase, the lower will be theamount of cold work required to achieve the desired effect. The amountof .cold work in terms of reduction in cross sectional area may range upto about 30% and generally from about 10 to 25%. 'llhe working may beachievedby cold drawing, cold rolling,I cold swaging, shot ing, ,orother forms of working in which a metal is Led beyond itsl elastic limitat room or aboveroom temperature at whcli recrystallization does notnormally perature of the matrix. Thus, it is possible to overcomeV anyweakness developed in a structural element due to localized overheatingmerely by peening the affected area.

A Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to Y without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand, such modifications and variations areconsidered to be withinthe purview and scope of the invention and the `appended claim. i

What is claimed is:

A method of fabricating a wrought metal product at i elevatedtemperatures which tend to impair the physical properties of at least aportion of the metal product being fabricated, such as occurs inwelding, and at fabrication temperatures in excess of the recovery andrecrystallizat tion temperature of the metal product which comprises,providing a metalproduct to be fabricated Vcomprising a metal selectedfrom the group consisting of Fe, Ni, Co, Fe-base, Ni-base and Co-basealloys; Cu, Ag, Au, Cubase, Ag-base `and`Aubase alloys; Pt, Pd, Ru, Rh,Ir

. and Pt-base. Pd-base, Ru-base, Rh-base and Ir-base alloys;

of over 90.000 calories per gram atom of oxygen. For

25 C. of about 96,200, A1203 of about 125,590, MgO of about 136,130, BeOof about 139,000, etc.

i `The amounts of the disperse phase employed may preferably range fromabout 0.1 to 15 vol. percent, preferably from about 1 to 12 vol.percent, and more preferably from about 3 to 10 vol. percent of thealloy. Where 7 0.1 to 4 vol. percent. I also prefer `that the particlesize obtained by internal oxidation range from about to 7 00 A. averagediameter.

4 Broadly speaking, the particle size of the disperse phase should notexceed 0.3 micron and preferably should range i i example SiO, has anegative free energy of formation at and` Mg, Zn, Cd, Al, Pb, SnandMg-base, Zn-ba`se, `Cdbase, Al-base, fPbbase and Sn-base alloys, havingdispersed substantially uniformly therethrough about 0.1 to 15 volumepercent of a stable insoluble disperse refractory oxide phasecharacterized by a negative free energy of formation at about25 C. of atleast about 90,000 calories per gram atom of oxygen and a particle sizenot exceeding about 0.3` micron, fabricating` at least a portion of saidmetal product containing said disperse refractory oxide phase at'anelevated fabrication temperature which tends to impair the physicalproperties of said portion, and then subjecting said fabricated portionto strain hardening, the amount of strain hardening being inverselyrelated to the volume of the disperse refractory oxide phase present,whereby at least said strain hardened 0 portion is characterized` byimproved high temperature strength properties equal to or above theoriginal properties prior to impairment thereof by high temperatureheating.

Y (References on followhlg page) lby 30 to Z50-times

